Beam-steered microphone arrays are in common usage, as in telephone conferencing systems. For example, electronic circuitry steers a beam toward each of several talking conference participants, to capture the participant's speech, and to reduce capture of (1) the speech of other participants, and (2) sounds originating from nearby locations. To facilitate understanding of the Invention, a brief description of some of the basic principles involved in beam steering will first be given.
The left side of FIG. 1 shows (1) an acoustic SOURCE which produces an acoustic signal 3, and (2) four omni-directional microphones M1–M4 which receive the signal 3.
The right side of FIG. 1 shows that the signal does not reach the microphones M at the same time. Rather, the signal reaches M1 first, and M4 last, because M4 is farthest away. The delays in reaching the microphones are labeled as D1, D2, and D3.
FIG. 2, left side, shows delay D3 resulting from the longer distance. If, on the right side of the Figure, an artificial delay D3, produced by circuit C, is added electronically to the output of microphone M1, then the outputs of M1 and M4 both require a time of (T+D3) to reach the summer SUM. That is, an actual delay D3 exists, and an artificial delay D3 is introduced, as indicated. Both microphone outputs now reach the summer SUM at the same time. The summer SUM produces output SUM1.
Similar delays D2 and D3 are applied to the outputs of microphones M3 and M2, respectively, causing them to reach summer SUM simultaneously also.
Consequently, because of the artificial delays introduced, the four signals, produced by the four microphones, reach the summer SUM simultaneously. Since the four signals arrive simultaneously, they are inphase. Thus, they all add together.
For example, if the signal produced by the SOURCE is a sine wave, such as (A sin t), the output of the summer SUM will be 4(A sin t). THEREFORE, in effect, the signal produced by the SOURCE has been amplified, by a gain of four.
It can be easily shown that, if the SOURCE moves to another position, the gain of four produced by the summer SUM will no longer exist. A smaller gain will be produced. Thus, the particular set of gains shown, namely the set (zero, D1, D2, and D3), will preferentially
amplify sound sources located at the location of the SOURCE shown in FIG. 2, compared with sources at other locations. The preferential amplification effectively suppresses sound emanating from other locations.
If the delays are kept the same, but re-arranged, as in FIG. 3, a mirror-image situation is created. Now the sound emanating from SOURCE 1 is preferentially amplified. Centerline 5 acts as the mirror.
In general, a collection 7 of the appropriate sets of delays will allow selective amplification of sources, at different positions, as in FIG. 4. To selectively amplify a given source, the appropriate set of delays is selected, and used.
In actual practice, the selective amplification is not as precise as the Figures would seem to indicate. That is, the selective amplification does not focus on a single, geometric point or spot, and amplify sounds emanating from that point exclusively. One reason is that the summations discussed above are valid only at a single frequency. In reality, sound sources transmit multiple frequencies. Another reason is that the microphones are not truly omni-directional. Thus, for these, and other reasons, the selective amplification occurs over cigar-shaped regions, termed “lobes.” FIG. 5 illustrates lobes L1–L5.
The lobes must be correctly understood. The lobes, as commonly used in the art, do not indicate that a sound source outside a lobe is blocked from being received. That is, the lobes do not map out cigar-shaped regions of space. Rather, the lobes are polar geometric plots. They plot signal magnitude against angular position. FIG. 6 provides an example.
The left side of the Figure shows a polar coordinate system, in which every point existing on the lobe, or plot P (such as points A and B on the right side) indicates (1) a magnitude and (2) an angle. (“Angle” is not an acoustic phase angle, but physical angle of a sound source, with respect to the microphone array, which is taken to reside at the origin.) The right side of the Figure shows two sound sources, A and B. As indicated, source A is located at 45 degrees. Its relative magnitude is about 2.8. Source B is located at about 22.5 degrees. Its relative magnitude is about 1.0.
Thus, the Figure indicates that source A will be amplified by 2.8. Source B will be amplified by 1.0.
Point D in FIG. 6 would appear to lie outside the plot. However, point D is “illegal.” The reason is that, again, the plot P is polar. Point D represents an angle, which is 45 degrees. The system gain at that angle is already represented by point A, which is on the plot P. Point D does not exist, for this system.
Restated, point D cannot be used to represent a source. If a source existed at the angle occupied by point D, then point A would indicate the gain with which the system would process that source.
One problem with beam-steered systems is that a noise source, such as an air conditioner or idling delivery truck, can exist within the lobe along with a talking person. The person's speech, as well as the noise, will be picked up.